The use of long chains of cascaded amplifiers and spatial filters for laser systems is well known. This system architecture, commonly known as a master-oscillator power amplifier (MOPA), is well characterized and widely used for large, high-power laser systems.
Experience with this technique has shown that large aperture amplifiers, namely the amplifier found at the output of a high power MOPA, are the most cost effective. Therefore, it is desirable to configure a system utilizing only these amplifiers.
The use of a single large aperture amplifier is common in alternative architectures like the multipass and regenerative systems in which a laser beam passes a number of times through the same laser amplifying medium. In these multipass systems, the beam passes through any given part of the gain medium more than once. A regenerative system is a special type of multipass system.
The present invention is concerned with controlled amplification of laser signals. The prior art discloses a number of oscillators using unstable ring laser resonators, such as U.S. Pat. No. 3,824,487 to Buczek, et al.; U.S. Pat. No. 4,135,787 to McLafferty; U.S. Pat. No. 4,239,341 to Cason, III, et al.; and U.S. Pat. No. 4,267,524 to Paxton, et al. All of these patents disclose unstable ring resonator structures having a lasing medium in the cavity. All of these references are concerned with the generation of a laser signal, that is, they disclose oscillators, rather than amplifiers as disclosed by the present invention. These references do not amplify an injected pulse and utilize the entire gain medium in a MOPA configuration.
U.S. Pat. No. 3,942,127 to Fluhr, et al., shows a reflective resonator without spatial filtering which multipasses the gain medium.
U.S. Pat. No. 3,753,147 to Schulthess shows an oscillator/amplifier configuration in which the beam makes only a single pass through the amplifying resonator.
U.S. Pat. No. 4,001,705 to Sinclair, et al., discloses a scanning device for a laser beam. The beam is deflected so as to horizontally and vertically scan a pattern. The system does not utilize an unstable resonator. The beam is not expanded and beam energy in a single pulse is not significantly increased.
U.S. Pat. Nos. 3,508,166 to Simmons, et al; 3,873,942 to Reilly; 4,011,523 to Mansell, et al.; 4,079,340 to Weiner, et al.; 4,126,381 to Chodzko, et al.; 4,190,814 to Van Workum; 4,320,359 to Peterson, et al., show various oscillator designs.